Method and apparatus to reduce charge sharing in pixellated energy discriminating detectors

Abstract

A CT detector includes a plurality of metallized anodes with each metallized anode separated from another metallized anode by a gap. A direct conversion material is electrically coupled to the plurality of metallized anodes and has a charge sharing region in which an electrical charge generated by an x-ray impinging the direct conversion material is shared between at least two of the plurality of metallized anodes. An x-ray attenuating material is positioned to attenuate x-rays directed toward the charge sharing region.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates generally to diagnostic imaging and, more particularly, to a direct conversion detector capable of providing photon count and / or energy data with reduced charge sharing between pixels of the direct conversion detector.[0002]Typically, in radiographic imaging systems, such as x-ray and computed tomography (CT), an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-rays. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signa...

AI Technical Summary

Benefits of technology

[0014]Therefore, it would be desirable to design a CT apparatus and met...

Problems solved by technology

A drawback of such detectors however is their inability to provide data or feedback as to the number and / or energy of photons detected.

Under the charge integration operation mode, the photodiode is not capable of discriminating between the energy level or the photon count from the scintillation.

In a typical imaging application, x-rays are absorbed in the direct conversion material which results in creation of an electrical charge in the direct conversion material.

A drawback of direct conversion semiconductor detectors, however, is that x-rays absorbed in the direct conversion material near the gaps or perimeters of the anodes can result in a charge being generated therein that is shared by at least two neighboring pixel anodes.

When using charge integration electronics, charge sharing can manifest itself as crosstalk between neighboring pixels, thus rendering the electronics susceptible to electronic noise amplification and spatial blurring of the image.

When using pulse counting electronics, charge sharing can result in dividing the charge between at least two anodes, resulting in lost counts when the amplitude of the charge pulse collected in at least one of the anodes is below a discrimination threshold.

Additionally, when pulse counting, high energy x-rays can result in loss of detection quantum efficiency (DQE) by the creation of two or more counts being collected in two or more neighboring anodes, thus mis-counting the events and binning, for instance, a single high energy event as two or more low-energy events.

The mis-counting of events and binning with respect to energy will degrade the capability for material discrimination.

Another drawback of direct conversion semiconductor detectors with regard to CT imaging is that the response at the edge and corners of the direct conversion crystal is not reproducible.

The changing internal field can cause a poor detector response that can lead to a non-optimal image.

Another drawback of direct conversion semiconductor detectors with regard to CT imaging is that these types of detectors cannot count at the very high x-ray photon flux rates typically encountered with conventional CT systems, e.g., at or above 5-100 million counts per sec per millimeter squared (Mcps).

The very high x-ray photon flux rate causes pile-up and polarization, which ultimately can lead to detector saturation.

Above these thresholds, the detector response is not predictable or has degraded dose utilization.

However, the bowtie filter may not be optimal given that a subject population is significantly less than uniform and not exactly elliptical in shape.

Low x-ray flux in the image projection tends to increase noise in the reconstructed image of the subject.

Detector saturation causes loss of imaging information and results in artifacts in x-ray projection and CT images.

In addition, hysteresis and other non-linear effects occur at flux levels near detector saturation as well as at flux levels over detector saturation.

Direct conversion detectors are susceptible to a phenomenon called “polarization,” where charge trapping inside the material changes the internal electric field, alters the detector count and energy response in an unpredictable way, and results in hysteresis where response is altered by previous exposure history.

However, smaller pixel size can result in higher cost because there are more channels per unit area which need to be connected to readout electronics.

Because the perimeters of the pixels is the region where a charge may be shared between two or more pixels, incomplete energy information and / or a miscount of x-ray photons occurs for such a charge because the readout electronics are not configured to combine near-simultaneous signals in neighboring pixels.

However, such electronics can be costly and difficult to implement.

A time-coincidence circuit would also not adequately preserve energy information about the x-ray event shared between two or more pixels without suffering degradation due to chance coincidence occurring with the near simultaneous arrival of two or more photons in neighboring regions.

However, a guard ring does not prevent trapping of charge within the semiconductor, and a guard ring does not prevent a changing electric field from developing within the semiconductor.

Images